ALBERT

Description: Synthetic aperture radar (SAR) images of the Greenland ice sheet collected by an airborne system clearly reveal the four melting facies of this sheet defined 30 years ago from snow stratigraphy studies by glaciologists. In particular, the radar echoes from the percolation facies have radiometric and polarimetric characteristics that are unique among terrestrial surfaces, but that resemble the exotic radar echoes recorded from the icy Galilean satellites. There, the radar signals interact with subsurface, massive ice features created in the cold, dry snow by seasonal melting and refreezing events. The subsurface features act as efficient reflectors of the incident radiation most likely via internal reflections. In the soaked-snow facies, the radar reflectivity is much lower because radar signals are attenuated by the wetter snow before they can interact with subsurface structures. Inversion algorithms to derive geophysical information from the SAR data are developed in both cases to estimate snow wetness in the soaked-snow facies and the mass of ice water retained in the percolation facies.

Description: Calibration of polarimetric radar systems is a field of research in which great progress has been made over the last few years. POLCAL (Polarimetric Radar Calibration) is a software tool intended to assist in the calibration of Synthetic Aperture Radar (SAR) systems. In particular, POLCAL calibrates Stokes matrix format data produced as the standard product by the NASA/Jet Propulsion Laboratory (JPL) airborne imaging synthetic aperture radar (AIRSAR). POLCAL was designed to be used in conjunction with data collected by the NASA/JPL AIRSAR system. AIRSAR is a multifrequency (6 cm, 24 cm, and 68 cm wavelength), fully polarimetric SAR system which produces 12 x 12 km imagery at 10 m resolution. AIRSTAR was designed as a testbed for NASA's Spaceborne Imaging Radar program. While the images produced after 1991 are thought to be calibrated (phase calibrated, cross-talk removed, channel imbalance removed, and absolutely calibrated), POLCAL can and should still be used to check the accuracy of the calibration and to correct it if necessary. Version 4.0 of POLCAL is an upgrade of POLCAL version 2.0 released to AIRSAR investigators in June, 1990. New options in version 4.0 include automatic absolute calibration of 89/90 data, distributed target analysis, calibration of nearby scenes with calibration parameters from a scene with corner reflectors, altitude or roll angle corrections, and calibration of errors introduced by known topography. Many sources of error can lead to false conclusions about the nature of scatterers on the surface. Errors in the phase relationship between polarization channels result in incorrect synthesis of polarization states. Cross-talk, caused by imperfections in the radar antenna itself, can also lead to error. POLCAL reduces cross-talk and corrects phase calibration without the use of ground calibration equipment. Removing the antenna patterns during SAR processing also forms a very important part of the calibration of SAR data. Errors in the processing altitude or in the aircraft roll angle are possible causes of error in computing the antenna patterns inside the processor. POLCAL uses an altitude error correction algorithm to correctly remove the antenna pattern from the SAR images. POLCAL also uses a topographic calibration algorithm to reduce calibration errors resulting from ground topography. By utilizing the backscatter measurements from either the corner reflectors or a well-known distributed target, POLCAL can correct the residual amplitude offsets in the various polarization channels and correct for the absolute gain of the radar system. POLCAL also gives the user the option of calibrating a scene using the calibration data from a nearby site. This allows precise calibration of all the scenes acquired on a flight line where corner reflectors were present. Construction and positioning of corner reflectors is covered extensively in the program documentation. In an effort to keep the POLCAL code as transportable as possible, the authors eliminated all interactions with a graphics display system. For this reason, it is assumed that users will have their own software for doing the following: (1) synthesize an image using HH or VV polarization, (2) display the synthesized image on any display device, and (3) read the pixel locations of the corner reflectors from the image. The only inputs used by the software (in addition to the input Stokes matrix data file) is a small data file with the corner reflector information. POLCAL is written in FORTRAN 77 for use on Sun series computers running SunOS and DEC VAX computers running VMS. It requires 4Mb of RAM under SunOS and 3.7Mb of RAM under VMS for execution. The standard distribution medium for POLCAL is a .25 inch streaming magnetic tape cartridge in UNIX tar format. It is also available on a 9-track 1600 BPI magnetic tape in DEC VAX FILES-11 format or on a TK50 tape cartridge in DEC VAX FILES-11 format. Other distribution media may be available upon request. Documentation is included in the price of the program. POLCAL 4.0 was released in 1992 and is a copyrighted work with all copyright vested in NASA.

Description: In June 1991, the NASA/Jet Propulsion Laboratory airborne synthetic-aperture radar (AIRSAR) instrument collected the first calibrated data set of multifrequency, polarimetric, radar observations of the Greenland ice sheet. At the time of the AIRSAR overflight, ground teams recorded the snow and firn (old snow) stratigraphy, grain size, density, and temperature at ice camps in three of the four snow zones identified by glaciologists to characterize four different degrees of summer melting of the Greenland ice sheet. The four snow zones are: (1) the dry-snow zone, at high elevation, where melting rarely occurs; (2) the percolation zone, where summer melting generates water that percolates down through the cold, porous, dry snow and then refreezes in place to form massive layers and pipes of solid ice; (3) the soaked-snow zone where melting saturates the snow with liquid water and forms standing lakes; and (4) the ablation zone, at the lowest elevations, where melting is vigorous enough to remove the seasonal snow cover and ablate the glacier ice. There is interest in mapping the spatial extent and temporal variability of these different snow zones repeatedly by using remote sensing techniques. The objectives of the 1991 experiment were to study changes in radar scattering properties across the different melting zones of the Greenland ice sheet, and relate the radar properties of the ice sheet to the snow and firn physical properties via relevant scattering mechanisms. Here, we present an analysis of the unusual radar echoes measured from the percolation zone.

Description: The Spaceborne Imaging Radar-C, X-Band Synthetic Aperture Radar (SIR-C/X-SAR) was launched on space shuttle Endeavour at 7:05 AM EDT, Saturday, April 9, 1994. Soon after launch, the radars were activated and began around the clock operations which lasted for the next 10 days.

Description: In this paper, we review the state of the art in imaging radar polarimetry, examine current developments in sensor technology and implementation for recording polarimetric measurements, and describe techniques and areas of application for the new remote sensing data.

Description: In June 1991, the NASA/JPL airborne SAR (AIRSAR) acquired C- (lambda = 5.6cm), L- (lambda = 24cm), and P- (lambda = 68m) band polarimetric SAR data over the Greenland ice sheet. These data are processed using version 3.55 of the AIRSAR processor which provides radiometrically and polarimetrically calibrated images. The internal calibration of the AIRSAR data is cross-checked using the radar response from corner reflectors deployed prior to flight in one of the scenes. In addition, a quantitative assessment of the noise power level at various frequencies and polarizations is made in all the scenes. Synoptic SAR data corresponding to a swath width of about 12 by 50 km in length (compared to the standard 12 x 12 km size of high-resolution scenes) are also processed and calibrated to study transitions in radar backscatter as a function of snow facies at selected frequencies and polarizations. The snow facies on the Greenland ice sheet are traditionally categorized based on differences in melting regime during the summer months. The interior of Greenland corresponds to the dry snow zone where terrain elevation is the highest and no snow melt occurs. The lowest elevation boundary of the dry snow zone is known traditionally as the dry snow line. Beneath it is the percolation zone where melting occurs in the summer and water percolates through the snow freezing at depth to form massive ice lenses and ice pipes. At the downslope margin of this zone is the wet snow line. Below it, the wet snow zone corresponds to the lowest elevations where snow remains at the end of the summer. Ablation produces enough meltwater to create areas of snow saturated with water, together with ponds and lakes. The lowest altitude zone of ablation sees enough summer melt to remove all traces of seasonal snow accumulation, such that the surface comprises bare glacier ice.

Description: The objective is to establish an empirical relationship between measurements of radar, aeolian, and surface roughness on a variety of natural surfaces and to understand the underlying physical causes. This relationship will form the basis for developing a predictive equation to derive aeolian roughness from radar backscatter. Results are given from investigations carried out in 1989 on the principal elements of the project, with separate sections on field studies, radar data analysis, laboratory simulations, and development of theory for planetary applications.